The present invention relates to a method and apparatus for inspecting defects of a specimen. More specifically, the present invention relates to a method and apparatus for inspecting defects of a specimen that is suitable for use in detecting optical images of fine patterns formed on transparent films of a semiconductor wafer.
Optical defect inspection apparatuses are equipped with microscope systems with high-precision auto-focus functions to provide optical imaging of fine patterns. This type of technology for microscope systems is presented in Japanese laid-open patent publication number Hei 6-265773 (xe2x80x9cAuto-focus apparatus for microscopesxe2x80x9d). The conventional technology described in this publication uses a TTL (Through The Lens) system where an auto-focus beam passes through the objective lens.
In TTL systems, both the imaging beam and the auto-focus beam pass through the objective lens. As a result, a high correlation between auto-focus detection results and imaging contrast is provided even when there is variation in the focal distance due to slight environmental changes such as temperature changes. Thus, TTL systems are standard in auto-focus systems used in microscope systems for detecting fine patterns formed on semiconductor wafers.
Currently, 0.18 micron widths are dominant as the pattern rule for semiconductor devices. This is expected to shift to 0.13 micron widths in the future. To form such fine circuit patterns, CMP (Chemical Mechanical Polishing) becomes necessary.
FIG. 4 shows cross-section drawings of a CMP-processed wafer. FIG. 4(a) shows a wafer with a lower-layer circuit pattern 1 formed. FIG. 4(b) shows an inter-layer insulative film 2, a transparent layer, formed on the lower-layer circuit pattern 1. The inter-layer insulative film 2 forms a mount where the lower-layer circuit pattern 1 is formed, resulting in unevenness in the inter-layer insulative film surface. This unevenness must be flattened out in order to allow a fine pattern to be formed on the inter-layer insulative film 2. This is because the exposure apparatus used to form the fine patterns has a shallow depth of focus, which prevents the unevenness from being ignored.
The flattening of this unevenness is performed with CMP processing. More specifically, a polishing pad is pressed against the surface of the inter-layer insulative film (transparent film) 2 and the surface is polished while a chemical abrasive (slurry) is applied. This provides a flat surface 3 as shown in FIG. 4(c).
A fine circuit pattern 4 can be formed on this flat surface 3, as shown in FIG. 4(d). With current and future semiconductor products with pattern widths of 0.18-0.13 microns, fine circuit patterns will be formed on CMP-processed transparent film. For this reason, defect inspection apparatuses need to provide optical features that allow precise detection of these circuit patterns.
However, an issue comes up regarding the auto-focal position in TTL systems for CMP-processed transparent films. This will be described with references to FIG. 5 and FIG. 6. FIG. 5(a) shows the angle of divergence xcex8 of an objective lens 5. The divergence angle xcex8 and the numerical aperture NA have the following relationship:
NA=sin xcex8xe2x80x83xe2x80x83Equation (1)
FIG. 5(b) illustrates the relationship indicated in Equation (1). As FIG. 5(b) shows, the divergence angle xcex8 is about 72 degrees even at NA=0.95, the setting closest to the NA that would be used under atmospheric pressure.
FIG. 6 shows the relationship between the angle of incidence i of a beam entering a transparent film and the reflectance R. Black circles indicate S-polarized light and black squares indicate P-polarized light. Reflectance R is a function of the angle of incidence i and changes depending on the polarization state. As the figure shows, even with the better S-polarized light, at 72 degrees (value 6 in the figure), there is only approximately 30% reflectance (value 7 in the figure), with the remaining 70% being transmitted into the transparent film. Thus, almost all of a TTL auto-focus beam will be transmitted into the transparent film and reflected from the lower-layer pattern before it is detected.
FIG. 7 shows a beam 8 illuminated from a TTL system passing through the transparent inter-layer insulative film 2 and focused on a surface 10 of a lower-layer circuit pattern 1. As a result, the object of detection, i.e., the fine pattern 4 on the transparent insulative film 2 and defects 11 formed on this layer, are defocused while the focus is on items not intended for detection, i.e., the lower-layer circuit pattern 1 formed below the transparent inter-layer insulative film 2 and defects 12 formed on this layer.
In semiconductor device defect inspections, identifying the layer at which a detected defect is present is important. Only by detecting defects by layer can prevention measures be taken to reduce defects. However, as shown in FIG. 7, if the detection beam is focused on a position other than the intended position, the accuracy of defect detection for that position is decreased drastically.
The present invention provides a defect inspection method and apparatus that is capable of detecting defects by using images captured at high resolutions with precise focus on the surface of a transparent film. This allows defects formed on fine patterns on the thin film to be detected in a highly sensitive manner.
The present invention also provides stable output unaffected by environmental temperature changes of images captured at high resolutions with precise focus on the surface of a transparent film.
The present invention is equipped with the technical means described below.
An inspection apparatus for a specimen on which a plurality of patterns intended to have identical shapes are arranged in a uniform manner includes: an imaging optical system with a relationship between illumination wavelength and objective lens numerical aperture that provides a resolution of no more than 0.18 microns, or preferably no more than 0.13 microns; an opto-electric converter disposed at an imaging position of the imaging optical system; an auto-focus optical system formed with an optical path disposed separate from the imaging optical system, with illumination applied at an incident angle of at least 85 degrees, preferably at least 88 degrees; means for adjusting a focal position of the imaging optical system based on a detection signal from the auto-focus optical system; and means for processing electronic signals from the opto-electrical converter.
According to another aspect of the present invention, the defect inspection apparatus also includes: means for detecting temperature; means for storing a relationship between temperature and focal position offset measured ahead of time or calculated through simulations; means for predicting a focal position offset using the relationship between temperature and focal position offset and based on temperature detection results from temperature detecting means; and means for correcting focal position offsets based on the prediction from this means.
Another aspect of the present invention, a defect inspection apparatus includes: a focal position measurement optical system formed by splitting an imaging optical system; a specimen, separate from the item on which detection is to be performed, serving as an object of measurement by the focal position measurement optical system; and means for measuring a focal position offset on the specimen using the focal position measurement optical system and adjusting an offset for the auto-focus optical system disposed separate from the imaging optical system based on these measurement results.